Method of Detecting and Handling and Endless RLC Restransmission

ABSTRACT

Disclosed is a radio (wireless) communication system providing a radio communication service and a terminal, and more particularly, to a method of effectively detecting and handling endless RLC retransmission so as to prevent endless RLC re-transmission occurring between the terminal and a network in an Evolved Universal Mobile Telecommunications System (E-UMTS) that has evolved from a Universal Mobile Telecommunications System (UMTS) or a Long Term Evolution (LTE) system.

TECHNICAL FIELD

The present invention relates to a radio (wireless) communication systemproviding a radio communication service and a terminal, and moreparticularly, to a method of effectively detecting and handling endlessRLC retransmission so as to prevent endless RLC retransmission occurringbetween the terminal and a network in an Evolved Universal MobileTelecommunications System (E-UMTS) that has evolved from a UniversalMobile Telecommunications System (UMTS) or a Long Term Evolution (LTE)system.

BACKGROUND ART

FIG. 1 shows an exemplary network structure of a Long-Term Evolution(LTE) system as a mobile communication system to which a related art andthe present invention are applied. The LTE system is a system that hasevolved from the existing UMTS system, and its standardization work iscurrently being performed by the 3GPP standards organization.

The LTE network can roughly be divided into an Evolved UMTS TerrestrialRadio Access Network (E-UTRAN) and a Core Network (CN). The E-UTRAN isgenerally comprised of a terminal (i.e., User Equipment (UE)), a basestation (i.e., Evolved Node B (eNode B)), an access gateway (aGW) thatis located at an end of the network and connects with one or moreexternal networks. The access gateway may be divided into a part thathandles processing of user traffic and a part that handles controltraffic. In this case, the access gateway part that processes the usertraffic and the access gateway part that processes the control trafficmay communicate with a new interface. One or more cells may exist in asingle eNB. An interface may be used for transmitting user traffic orcontrol traffic between eNBs. The CN may include the aGW and a node orthe like for user registration of the UE. An interface fordiscriminating the E-UTRAN and the CN may be used.

FIGS. 2 and 3 show respective exemplary structures of a radio interfaceprotocol between the terminal and the E-UTRAN based on the 3GPP radioaccess network standards. The radio interface protocol has horizontallayers comprising a physical layer, a data link layer, and a networklayer, and has vertical planes comprising a user plane (U-plane) fortransmitting user data information and a control plane (C-plane) fortransmitting control signaling. The protocol layers in FIGS. 2 and 3 canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on three lower layers of an open system interconnection(OSI) standard model widely known in the communication system. The radioprotocol layers exist as pairs between the UE and the E-UTRAN and handlea data transmission in a radio interface.

The layers of the radio protocol control plane in FIG. 2 and those ofthe radio protocol user plane in FIG. 3 will be described as follows.

The physical layer, the first layer, provides an information transferservice to an upper layer by using a physical channel. The physicallayer is connected to an upper layer called a medium access control(MAC) layer via a transport channel. Data is transferred between the MAClayer and the physical layer via the transport channel. The transportchannel is divided into a dedicated transport channel and a commontransport channel according to whether or not a channel is shared.Between different physical layers, namely, between a physical layer of atransmitting side and that of a receiving side, data is transmitted viathe physical channel using radio resources.

The second layer includes various layers. First, a medium access control(MAC) layer performs mapping various logical channels to varioustransport channels and performs logical channel multiplexing by mappingseveral logical channels to a single transport channel. The MAC layer isconnected to an upper layer called a radio link control (RLC) layer by alogical channel. The logical channel is roughly divided into a controlchannel that transmits information of the control plane and a trafficchannel that transmits information of the user plane according to a typeof transmitted information.

A Radio Link Control (RLC) layer of the second layer segments and/orconcatenates data received from an upper layer to adjust the data sizeso as for a lower layer to suitably transmit the data to a radiointerface. In addition, in order to guarantee various Quality ofServices (QoSs) required by each radio bearer (RB), the RLC layerprovides three operational modes: a Transparent Mode (TM); anUnacknowledged Mode (UM); and an Acknowledged Mode (AM). In particular,the AM RLC performs a re-transmission function through an AutomaticRepeat and Request (ARQ) for a reliable data transmission.

A Packet Data Convergence Protocol (PDCP) layer of the second layerperforms a function called header compression that reduces the size of aheader of an IP packet, which is relatively large and includesunnecessary control information, in order to effectively transmit the IPpacket such as an IPv4 or IPv6 in a radio interface having a narrowbandwidth. The header compression increases transmission efficiencybetween radio interfaces by allowing the header part of the data totransmit only the essential information. In addition, the PDCP layerperforms a security function in the LTE system. The security functionincludes ciphering for preventing data wiretapping by a third party, andintegrity protection for preventing data manipulation by a third party.

The Radio Resource Control (RRC) layer located at the lowermost portionof the third layer is defined only in the control plane, and controls alogical channel, a transport channel and a physical channel in relationto the configuration, reconfiguration, and release of radio bearers(RBs). In this case, the RBs refer to a logical path provided by thefirst and second layers of the radio protocol for data transmissionbetween the UE and the UTRAN. In general, configuration (establishment,setup) of the RB refers to the process of stipulating thecharacteristics of a radio protocol layer and a channel required forproviding a particular data service, and setting the respective detailedparameters and operational methods. The RBs include two types: aSignaling RB (SRB) and a Data RB (DRB). The SRB is used as a path fortransmitting an RRC message on a C-plane, and the DRB is used as a pathfor transmitting user data on a U-plane.

Hereinafter, the RLC layer will be explained in more detail. Asmentioned above, the RLC layer operates in three modes, TM, UM, and AM.Since the RLC layer performs a simple function in the TM, only the UMand AM will be explained.

The UM RLC generates each Packet Data Unit (PDU) with a PDU headerincluding a Sequence Number (SN), thereby allowing a receiving side toknow which PDU has been lost while being transmitted. Accordingly, theUM RLC transmits broadcast/multicast data or transmits real-time packetdata such as voice (e.g., VoIP) of a Packet Service domain (PS domain)or streaming on a user plane. Also, on a control plane, the UM RLCtransmits, to a specific terminal or specific terminal group in a cell,an RRC message requiring no response for reception acknowledgement.

Similar to the UM RLC, the AM RLC generates each PDU with a PDU headerincluding a Sequence Number (SN). Differently from the UM RLC, in the AMRLC, a receiving side performs acknowledgement for PDUs transmitted froma sending side. In the AM RLC, the reason why the receiving sideperforms acknowledgement is to request the sending side to retransmit aPDU if the receiving side fails to receive the PDU. The re-transmissionfunction is the main characteristic part of the AM RLC. The AM RLC aimsto guarantee error-free data transmission using the re-transmissionfunction. To this end, the AM RLC handles transmission of non-real timepacket data such as TCP/IP of PS domain on the user plane, and transmitsan RRC message that necessarily requires a reception acknowledgementamong RRC message transmitted to a specific terminal in a cell on thecontrol plane.

In terms of directionality, the UM RLC is used for uni-directionalcommunications, while the AM RLC is used for bi-directionalcommunications due to feedback from the receiving side. The UM RLC isdifferent from the AM RLC in the aspect of configuration. The UM RLC andthe AM RLC are different in terms of structural aspect: the UM RLC isthat a single RLC entity has only one structure of transmission orreception but the AM RLC is that both a sending side and a receivingside exist in a single RLC entity.

The AM RLC is complicated due to its re-transmission function for data.The AM RLC is provided with a retransmission buffer as well as atransmission/reception buffer for retransmission management. The AM RLCperforms many functions, e.g., usage of a transmission/reception windowfor flow control, polling to request a status information (statusreport) from a receiving side of a peer RLC entity by a sending side, areceiving side's status report informing about its buffer status to asending side of a peer RLC entity, and generating of a status PDU totransmit status information, or the like. In order to support thosefunctions, the AM RLC requires to have various protocol parameters,status variables, and timers. The PDUs used for controlling datatransmission in the AM RLC, such as the status report, a status PDU, orthe like, are called Control PDUs, and the PDUs used for transferringuser data are called Data PDUs.

In the AM RLC, the RLC Data PDU is further divided into an AMD PDU andan AMD PDU segment. The AMD PDU segment has a portion of data belongingto the AMD PDU. In the LTE system, a maximum size of a data blocktransmitted by the terminal may vary at each transmission. For instance,having generated and transmitted an AMD PDU having a size of 200 bytesat a certain time period, a sending side AM RLC entity is required toretransmit the AMD PDU since it has received a NACK from a receivingside AM RLC. Here, if a maximum size of a data block which can beactually transmitted is assumed 100 bytes, the AMD PDU cannot beretransmitted in its original form. To solve this problem, the AMD PDUsegments are used. The AMD PDU segments refer to the AMD PDU dividedinto smaller units. During such process, the sending side AM RLC entitydivides the AMD DPU into the AMD PDU segments so as to transmit the sameover a certain period of time. Then, the receiving side AM RLC entitydecodes the AMD PDU from the received AMD PDU segments.

In the related art, the PDCP layer as an upper layer of the RLC has atimer for each PDCP SDU (Service Data Unit). If an ACK is not receiveduntil the timer expires, the PDCP layer discards the corresponding PDCPSDU and Protocol Data Unit (PDU), and simultaneously, commands the RLCto discard the corresponding PDCP PDU, i.e., the RLC SDU. Upon receivingthe RLC SDU discard indication, the AM RLC would discard the RLC SDU ifno segment of the RLC SDU has been mapped to an AMD PDU yet and thuslystored in the RLC buffer. However, if at least one segment of the RLCSDU has already been mapped to an AMD PDU, the AM RLC would not discardthe RLC SDU but retransmit the RLC SDU until an ACK is received.

In the related art, there is no restriction on the PDU retransmission bythe AM RLC, such as a maximum allowable time or frequency fortransmission, or the like. This is because the LTE system havingemployed technologies, such as OFDM, MIMO, HARQ and the like, assumesthat the transmission in the physical layer is stable. That is, sincethe physical layer has a very low transmission error rate, anyrestrictions on the retransmission in the RLC layer are not necessary.Therefore, the AM RLC in theory may perform endless retransmission.

For some reasons, however, the RLC may continually fail to retransmit.Among those, a variety of status parameters of the RLC protocol maymalfunction by a protocol error (residual error) that is not detectedeven by the Cyclic Redundancy Check (CRC) in the physical layer, or thereceiving side may continually discard the PDUs having successfully beentransmitted by the sending side due to different RLC implementationschemes between the terminal and the network. With such RLC-associatedproblems, there is a possibility to fail even if the retransmission isendlessly performed. Therefore, there is a need to have a solution forthe endless retransmission by the RLC.

DISCLOSURE OF INVENTION Technical Solution

Therefore, an object of the present invention is to prevent an endlessPDU re-transmission by the AM RLC so as to prevent the RLC protocol froma deadlock situation.

For this, the present invention has proposed that the AM RLC restricts are-transmission of a specific SDU or PDU including the SDU by aretransmission time or frequency, and if the retransmission is performeduntil it reaches to the restricted time or frequency, the AM RLCconsiders it as an endless retransmission situation, thus to inform sucherror to the upper RRC layer. In particular, when limiting the SDU orPDU retransmission by a certain time or frequency, a timer or counter isnot started for all RLC SDUs or AMD PDUs. Instead, the timer or counteris started only for an RLC SDU having received a discard indication bythe PDCP or an AMD PDU including the RLC SDU, thereby minimizing thenumber of timers or counters to be used.

To achieve this and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described herein,there is provided a method of retransmitting data unit between aterminal and a network in wireless communication system, the methodcomprising: determining if a counting condition is triggered, whereinthe counting condition is triggered when at least one portion of thedata unit is allocated with a radio link control (RLC) sequence number;and setting a counter for retransmission of the data unit when thecounting condition is triggered.

To achieve this and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described herein,there is also provided a method of retransmitting data unit between aterminal and a network in wireless communication system, the methodcomprising: determining if a timer starting condition is triggered,wherein the timer starting condition is triggered when at least oneportion of the data unit is allocated with a radio link control (RLC)sequence number; and setting a timer for retransmission of the data unitwhen the timer starting condition is triggered.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an exemplary network structure of an Evolved UniversalTerrestrial Radio Access Network (E-UTRAN) as a mobile communicationsystem to which a related art and the present invention are applied;

FIG. 2 is an exemplary view of related art control plane architecture ofa radio interface protocol between a terminal and an E-UTRAN;

FIG. 3 is an exemplary view of related art user plane architecture of aradio interface protocol between a terminal and an E-UTRAN; and

FIG. 4 is an exemplary view of a method of detecting an endless RLCre-transmission according to the present invention.

MODE FOR THE INVENTION

One aspect of this disclosure relates to the recognition by the presentinventors about the problems of the related art as described above, andfurther explained hereafter. Based upon this recognition, the featuresof this disclosure have been developed.

Although this disclosure is shown to be implemented in a mobilecommunication system, such as a UMTS developed under 3GPPspecifications, this disclosure may also be applied to othercommunication systems operating in conformity with different standardsand specifications.

Hereinafter, description of structures and operations of the preferredembodiments according to the present invention will be given withreference to the accompanying drawings.

As described above, the present invention aims to prevent an endless PDUre-transmission by the AM RLC so as to prevent the RLC protocol from adeadlock situation.

To this end, the present invention has proposed that the AM RLCrestricts a re-transmission of a specific SDU or PDU including the SDUby a retransmission time or frequency, and if the retransmission isperformed until it reaches to the restricted time or frequency, the AMRLC considers it as an endless retransmission situation, thus to informsuch error to the upper RRC layer. In particular, when limiting the SDUor PDU retransmission by a certain time or frequency, a timer or counteris not started for all RLC SDUs or AMD PDUs. Instead, the timer orcounter is started only for an RLC SDU having received a discardindication by the PDCP or an AMD PDU including the RLC SDU, therebyminimizing the number of timers or counters to be used.

FIG. 4 is an exemplary view of a method of detecting an endless RLCre-transmission according to the present invention. Referring to FIG. 4,a method of detecting an endless retransmission error may now bedescribed in more detail.

First, descriptions of a method of detecting an endless retransmissionsituation using a timer will now be given. If the AM RLC receives adiscard indication for a specific RLC SDU from the upper layer PDCP, theAM RLC immediately discards the SDU if no segment of the RLC SDU hasbeen mapped to an AMD PDU yet. On the contrary, if at least one segmentof the RLC SDU has already been mapped to an AMD PDU, the AM RLC startsa timer such that if an ACK is not received until the timer expires, itis considered as an endless retransmission situation. Here, the timervalue is transferred through the RRC at the RB setup, and a timer isstarted for each SDU in which at least one segment of the SDU havingreceived a discard indication by the PDCP has already been mapped to anAMD PDU. And, a basis for determining which SDU is transmitted, in aperspective of the RLC, is that whether at least one segment of the SDUis included in the AMD PDU. This is because an RLC sequence number isadded when the AMD PDU is generated. Accordingly, if the PDU is nottransmitted to the receiving side RLC, a gap is generated in the RLC SNin the receiving side, which leads to a protocol error.

Such described method will be given in the following procedure text.

-   -   at RB setup, RRC gives a timer expiry value to RLC.    -   when PDCP indicates to RLC to discard a particular RLC SDU, the        RLC transmitter shall:    -   if no segment of the RLC SDU has been mapped to an AMD PDU yet:    -   discard the indicated RLC SDU;    -   else (at least one segment of the RLC SDU has already been        mapped to an AMD PDU):    -   start a timer for the RLC SDU (a timer is started for each RLC        SDU that the PDCP indicates to RLC to discard);    -   if the RLC transmitter does not receive ACK for the RLC SDU        until the timer expires, the RLC transmitter shall consider it        as an endless RLC retransmission situation.

Next, a method of detecting an endless retransmission situation using acounter is described. As another method of detecting the endlessretransmission situation, a maximum number of retransmission may beused. An operation method thereof is similar to that using the timerexcept that a counter, instead of a timer, is used. That is, if the AMRLC receives a discard indication for a specific RLC SDU from the upperlayer PDCP, the AM RLC immediately discards the SDU if no segment of theSDU has been mapped to an AMD PDU yet. On the contrary, if at least onesegment of the SDU has already been mapped to an AMD PDU, the AM RLCsets the counter so as to count the number of retransmission. If an ACKfor the SDU has not been received until the counter reaches the maximumnumber of retransmission, it is considered as an endless retransmissionsituation. Here, the maximum number of retransmission is transferredthrough the RRC at the RB setup, and the counter is set for each SDU inwhich at least one segment of the SDU having received a discardindication by the PDCP has already been mapped to an AMD PDU. Thecounter is incremented by 1 after each retransmission of the SDU. And, abasis for determining which SDU is transmitted, in the perspective ofthe RLC, is similar to that using the timer, that whether at least onesegment of the SDU is included in the AMD PDU.

Such described method will be given in the following procedure text.

-   -   at RB setup, RRC gives a maximum number of retransmission value        to RLC.    -   when PDCP indicates to RLC to discard a particular RLC SDU, the        RLC transmitter shall:    -   if no segment of the RLC SDU has been mapped to an AMD PDU yet:    -   discard the indicated RLC SDU.    -   else (at least one segment of the RLC SDU has already been        mapped to an AMD PDU):    -   set a state variable that counts the number of retransmission        for the RLC SDU to initial value. (A state variable is        configured for each RLC SDU that the PDCP indicates to RLC to        discard)    -   for each retransmission of the RLC SDU, the RLC transmitter        shall increment the state variable by one.    -   if the value of the state variable reaches the maximum value,        the RLC transmitter shall consider it as an endless        retransmission situation.

Once the endless RLC retransmission situation is detected with suchdescribed method, the endless RLC retransmission situation should besolved. Methods for solving the endless RLC retransmission situation mayinclude the following:

First, such situation may be solved by the RLC itself. That is, an RLCSDU discard and a MRW movement may be employed. The transmitting sideRLC discards the SDU having the endless retransmission situation, shiftsa starting point of a transmitting window to a SDU that has not beenfirstly received an ACK among the successive SDUs after the discardedSDU, and transmits a Move Receiving Window command to the receiving sideRLC so as to shift a starting point of a receiving window to a SDU thathas not been firstly received an ACK among the successive SDUs after thediscarded SDU. Alternatively, an RLC reset method may be employed, inwhich the transmitting side RLC is reset by itself so as to initializeall status parameters, timers or the like, and then transmits the resetindication to the receiving side RLC, thus to reset the receiving sideRLC.

In addition, a method using an RRC signaling may be used. That is, if anendless re-transmission situation occurs, the transmitting side RLCinforms such situation to its RRC, and the transmitting side RRC informsthe same to the receiving side RRC through an RRC signaling, thus tore-establish a corresponding RB (RB re-establishment). Alternatively, ifan endless retransmission situation occurs, the transmitting side RLCinforms such situation to its RRC, and the transmitting side RRCre-establishes an RRC connection, thus to re-establish all RBs (RRCconnection re-establishment).

Namely, the related art has not provided any solution for the endlessretransmission situation, causing the protocol to be in a deadlocksituation. The present invention proposes a solution using a timer or acounter so as to enable the wireless protocol to stably operate in amobile communication system. In particular, the timer or the counter isset to start under a specific condition only, thereby highly reducingthe number of the timers or counters to be used for the solution.

The present disclosure may provide a method of retransmitting data unitbetween a terminal and a network in wireless communication system, themethod comprising: determining if a counting condition is triggered,wherein the counting condition is triggered when at least one portion ofthe data unit is allocated with a radio link control (RLC) sequencenumber; and setting a counter for retransmission of the data unit whenthe counting condition is triggered, wherein the data unit is either RLCPDU (Protocol Data Unit) or RLC SDU (Service Data Unit), a radio linkcontrol (RLC) layer receives a discard indication to discard the dataunit from an upper layer, the RLC layer is operated in an AcknowledgedMode (AM), a data unit retransmission error is occurred when a number ofretransmission of the data unit exceeds a maximum retransmission numberassigned to the counter, the maximum retransmission number of thecounter is assigned by receiving a RRC message, and the counter isincremented by 1 after each retransmission of the data unit.

It can be also said that the present disclosure may provide a method ofretransmitting data unit between a terminal and a network in wirelesscommunication system, the method comprising: determining if a timerstarting condition is triggered, wherein the timer starting condition istriggered when at least one portion of the data unit is allocated with aradio link control (RLC) sequence number; and setting a timer forre-transmission of the data unit when the timer starting condition istriggered, wherein the data unit is either RLC PDU (Protocol Data Unit)or RLC SDU (Service Data Unit), a radio link control (RLC) layerreceives a discard indication to discard the data unit from an upperlayer, the RLC layer is operated in an Acknowledged Mode (AM), a dataunit retransmission error is occurred if the retransmission of the dataunit is not successfully received before an expiry of the timer, a timervalue assigned to the timer is received through a RRC message, the upperlayer is a PDCP (Packet Data Convergence Protocol) layer.

Although the present disclosure is described in the context of mobilecommunications, the present disclosure may also be used in any wirelesscommunication systems using mobile devices, such as PDAs and laptopcomputers equipped with wireless communication capabilities (i.e.interface). Moreover, the use of certain terms to describe the presentdisclosure is not intended to limit the scope of the present disclosureto a certain type of wireless communication system. The presentdisclosure is also applicable to other wireless communication systemsusing different air interfaces and/or physical layers, for example,TDMA, CDMA, FDMA, WCDMA, OFDM, EV-DO, Wi-Max, Wi-Bro, etc.

The exemplary embodiments may be implemented as a method, apparatus orarticle of manufacture using standard programming and/or engineeringtechniques to produce software, firmware, hardware, or any combinationthereof. The term “article of manufacture” as used herein refers to codeor logic implemented in hardware logic (e.g., an integrated circuitchip, Field Programmable Gate Array (FPGA), Application SpecificIntegrated Circuit (ASIC), etc.) or a computer readable medium (e.g.,magnetic storage medium (e.g., hard disk drives, floppy disks, tape,etc.), optical storage (CD-ROMs, optical disks, etc.), volatile andnon-volatile memory devices (e.g., EEPROMs, ROMs, PROMs, RAMs, DRAMs,SRAMs, firmware, programmable logic, etc.).

Code in the computer readable medium may be accessed and executed by aprocessor. The code in which exemplary embodiments are implemented mayfurther be accessible through a transmission media or from a file serverover a network. In such cases, the article of manufacture in which thecode is implemented may comprise a transmission media, such as a networktransmission line, wireless transmission media, signals propagatingthrough space, radio waves, infrared signals, etc. Of course, thoseskilled in the art will recognize that many modifications may be made tothis configuration without departing from the scope of the presentdisclosure, and that the article of manufacture may comprise anyinformation bearing medium known in the art.

As the present disclosure may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the metes and bounds of theclaims, or equivalents of such metes and bounds are therefore intendedto be embraced by the appended claims.

1. A method of retransmitting data unit between a terminal and a networkin wireless communication system, the method comprising: determining ifa counting condition is triggered, wherein the counting condition istriggered when at least one portion of the data unit is allocated with aradio link control (RLC) sequence number; and setting a counter forretransmission of the data unit when the counting condition istriggered.
 2. The method of claim 1, wherein the data unit is either RLCPDU (Protocol Data Unit) or RLC SDU (Service Data Unit).
 3. The methodof claim 1, wherein a radio link control (RLC) layer receives a discardindication to discard the data unit from an upper layer.
 4. The methodof claim 3, wherein the RLC layer is operated in an Acknowledged Mode(AM).
 5. The method of claim 1, wherein a data unit retransmission erroris occurred when a number of retransmission of the data unit exceeds amaximum retransmission number assigned to the counter.
 6. The method ofclaim 5, wherein the maximum retransmission number of the counter isassigned by receiving a RRC message.
 7. The method of claim 1, whereinthe counter is incremented by 1 after each re-transmission of the dataunit.
 8. A method of retransmitting data unit between a terminal and anetwork in wireless communication system, the method comprising:determining if a timer starting condition is triggered, wherein thetimer starting condition is triggered when at least one portion of thedata unit is allocated with a radio link control (RLC) sequence number;and setting a timer for retransmission of the data unit when the timerstarting condition is triggered.
 9. The method of claim 8, wherein thedata unit is either RLC PDU (Protocol Data Unit) or RLC SDU (ServiceData Unit).
 10. The method of claim 8, wherein a radio link control(RLC) layer receives a discard indication to discard the data unit froman upper layer.
 11. The method of claim 10, wherein the RLC layer isoperated in an Acknowledged Mode (AM).
 12. The method of claim 8,wherein a data unit retransmission error is occurred if theretransmission of the data unit is not successfully received before anexpiry of the timer.
 13. The method of claim 12, wherein a timer valueassigned to the timer is received through a RRC message.
 14. The methodof claim 10, wherein the upper layer is a PDCP (Packet Data ConvergenceProtocol) layer.